1. Field of the Invention
The invention relates to an infrared-sensitive planographic printing plate precursor and in particular to an infrared-sensitive planographic printing plate precursor resistant to scratching on its photosensitive layer when stacked.
2. Description of the Related Art
Amid recent remarkable progress in laser technology, higher-output and smaller-sized solid-state and semiconductor lasers that emit a laser beam in the near-infrared to infrared region have become more easily available. These lasers are particularly useful in the field of planographic printing as a light source during direct plate making from digital data from, for example, a computer.
The recording layer in a positive-type planographic printing plate precursor for direct plate making using such an infrared laser contains an alkali-soluble resin and an infrared absorbent absorbing light and generating heat as its essential components. The infrared absorbent and the alkali-soluble resin are dissolved in an alkaline developing solution in the exposed region (nonimage region), as the interaction between them is weakened by the heat generated by the infrared absorbent, while the infrared absorbent functions as a solubilization-suppressing agent reducing the solubility of the alkali-soluble resin by interaction with the alkali-soluble resin in the unexposed region (image region), giving an image. However, the mechanical strength of the recording layer in this positive-type planographic printing plate precursor is insufficient, and such a printing plate has the problem that there are defects on the plate surface, leading to image loss in the image region after development when the plate surface is rubbed intensely with various parts during production, processing, transportation of the plate, and handling of the plate surface.
To avoid such a problem, planographic printing plate precursors are generally packaged with an insert paper (partitioning paper) inserted between the plates. However, the insert paper leads to 1) increase costs and 2) problems in disposal, and thus, there exists a need for an insert paper-free process that does not require the insert paper. Recently, in particular, along with the popularization of CTP systems, there has been an increased trend toward installing an auto-loader supplying plate materials into an exposure device, and thus, there is an urgent need for an insert-paper-free process, to avoid the tedious labor of manually removing the insert paper in advance or the problem of scratching during removal of the insert paper even when an automatic insert-paper removing mechanism is installed.
A known method directed toward elimination of the insert paper is to prevent the rear face of a supporting plate from mechanically damaging the photosensitive layer due to contact of the photosensitive layer with the rear face of the supporting plate.
Proposed are, for example, photosensitive planographic printing plates having a coating layer of a resin having a glass transition temperature of 60° C. or higher selected from the group consisting of saturated copolymeric polyester resins, phenoxy resins, polyvinylacetal resins and vinylidene chloride copolymer resins on the face opposite to the photosensitive layer (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2005-62456), and photosensitive planographic printing plates having a rough-surfaced organic polymer layer on the face opposite to the photosensitive layer (see, for example, JP-A No. 2002-254843).
As described above, methods of using a backcoat layer of an organic polymer are effective to a certain degree in reducing the damage of the photosensitive layer.
However, if a hard coating layer is formed as in JP-A No. 2005-62456, or if a rough-surfaced organic polymer layer is formed as in JP-A No. 2002-254843, it was found that the photosensitive layer was vulnerable to the damage by adhesion of the photosensitive layer with the backcoat layer and to scratches by rubbing between the photosensitive layer and the backcoat layer, particularly when the plate materials are fed, from being stacked without insert paper into an auto-loader, feeding the plate materials automatically to the laser exposure machine, in a structure where the photosensitive layer and the rear face (coated layer or organic polymer layer) are pressed against each other.
In addition, such a planographic printing plate precursor having a backcoat layer and a recording layer relatively lower in strength containing an alkali-soluble resin and an infrared absorbent such as that described above was found to be vulnerable to scratching on the recording layer under load, when the planographic printing plate precursor is coated, dried, and cut into pieces in its production process or when the stacked plate precursors are fed into an auto-loader.
Alternatively, JP-A No. 2002-46363 discloses a recording material for offset printing having a radiation-sensitive layer and an organic polymer-containing backcoat layer that allows stacking without insert paper, which has a backcoat layer of an organic polymer having a glass transition temperature of 35° C. or higher containing a pigment such as silica gel. However, use of an inorganic pigment such as silica gel in the backcoat layer causes the problem of scratching on the photosensitive layer due to rubbing when the products are stacked, packaged, and transported without use of insert paper, because the inorganic pigment is very hard.
There has also been proposed a method of forming a matte on the face opposite to the photosensitive layer by electrostatic spraying (see, for example, JP-A No. 2003-63162). Although it has been reported that the plates could be stacked without insert paper, the plates in such a configuration often resulted in adhesion between plates when stored stacked without insert paper, especially in summer under a high-humidity environment.
The backcoat layer of such an organic polymer is lower in close contact with the supporting plate, and thus, the plate materials are rubbed by each other by vibration and the organic polymer thereon is occasionally exfoliated partially by the stress when the multiple plate materials are stored and transported as stacked. As a result, it is often difficult to reduce the mechanical damage of the photosensitive layer by contact between the photosensitive layer and the rear face of the supporting plate, causing a problem in obtaining an insert-paper-free process.